Quantitative assessment of weathering degree of the Touzhai rock-avalanche deposit in Southwest China.

Weathering is one of the main preparatory causal factors for rock-avalanche events and weathering assessment of involved materials can refine understanding of geomorphic processes of this class. The weathering degree of the deposit of the Touzhai basalt rock avalanche in China, is quantitatively evaluated herein through an integrated geochemical analysis, coupled with meso- and micro-scopic examination of fragments, which can help to understand the inhomogeneity of weathering in rock slopes. The clasts of 12 size classes of the debris making up the deposit, ranging from meter-sized boulder to fines of <25 μm, are assessed with 16 geochemical parameters and eight weathering indices. The results show that the debris can be subdivided into two distinct groups: fresh (or fresh-slightly weathered) and weathered (moderately-highly weathered). Group 1 includes five classes of several meters, 15-10 cm, 5-4 cm, 2-1 cm and 1-0.5 cm, and Group 2 covers a particle size range of <0.5 cm to <25 μm. Of special interest is that fresh debris accounts for at least ~80 wt.% of the rock-avalanche deposit. The unexpectedly significant proportion of fresh debris appears to be a necessary prerequisite for the rock-avalanche occurrence and can be attributed to the aphanitic texture and low permeability of the basalt. On the basis of the low bulk density and strength of saprolite (completely weathered rocks), the slopes containing ~ 80-90% of unweathered materials may fail as rock avalanche-debris flows. They may collapse as rockslide-debris flows if saprolite crusts continue to thicken. In addition, the slopes made entirely, of saprolite, would not host catastrophic failures. • The weathering of a rock-avalanche deposit in China is quantitatively assessed. • The inhomogeneity of weathering of rock masses is taken into account in this paper. • Surprisingly high proportion of fresh debris is found in the loose earthy deposit. • The weathered states of rock masses can control the style of slope failures. [ABSTRACT FROM AUTHOR]